test/make_dist: Drop --download-only error message
test/make_dist: Drop --download-only error message

There are other reasons than "does not exist" why the download was not
done, and for each case, download_dist() already prints out a specific
reason. So this message is redundant and confusing.

mvollmer/cockpit,cockpit-project/cockpit,garrett/cockpit,cockpit-project/cockpit,martinpitt/cockpit,mvollmer/cockpit,martinpitt/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,martinpitt/cockpit,garrett/cockpit,garrett/cockpit,martinpitt/cockpit,martinpitt/cockpit,mvollmer/cockpit,garrett/cockpit,mvollmer/cockpit,cockpit-project/cockpit,mvollmer/cockpit,garrett/cockpit